Did broadband support for a few years. It’s basically a signal on a line subject to interference. If you’re on standard ADSL then any electromagnetic interference between you and the exchange could cause the drops.
What should happen is the router and exchange should establish a connection that gives you the best speed for the line leaving a margin for error. If you get interference over the margin of error then the line drops. It should reconnect at a slower speed with more margin and stay stable.
The problem happens when the interference is really bad or really patchy. you may end up on a really basic banded profile where you get really poor speed to allow for a stable connection.
The problem is compounded more by how far from the exchange you are. There’s a limit to how far a broadband signal will travel before the line gets too noisy to maintain a consistent connection. Longer line means slower speed and more interference. As a rough guide if your overall speed is 2Mbps or less, chances are you’re on a longer line and drops might be just part of the experience.
ISPs should be able to see from the line stats what’s happening and errors should be a rough guide as to where the problem is. This is a good site that explains broadband better than I can:
http://www.kitz.co.uk/